The present invention relates to a sound field correcting device for providing a natural acoustic localization of acoustic outputs of the right and left channels in a narrow space such as inside of a motor vehicle.
A distance between two human ears, in connection with a wavelength of a sound wave reaching each ear, constitutes one of the major factors used in determining an acoustic space impression. A phase difference between the sound waves reaching both ears is greatly influenced by the low frequency components of the sound wave whose wavelength is substantially equal to the distance between the ears, and the sound wave has a unique directivity pattern. A man perceives an acoustic space impression on the basis of the difference of level and phase between the sound waves reaching the ears, directivity patterns of the sound wave, and the like.
A quantity representing an auditory correlation between the ears, an interaural correlation coefficient .rho.LR has been used, and is expressed by ##EQU1## where PL(t) and PR(t) are sound pressures applied to the right and left ears, PL(t) and PR(t) are time average values of PL(t) and PR(t).
When the interaural correlation coefficient .rho.LR approaches -1, the auditory perspective and extensity become smaller. At a value of approximately 0 (zero) of the coefficient .rho.LR, the auditory extensity becomes large. When the coefficient approaches +1, the auditory perspective becomes large.
Let us consider the equation (1) in a situation that in an ordinary listening room, a couple of speakers driven in phase are placed, and a listener is located at a position distanced equally from the speakers. In low and medium frequencies of sound the coefficient .rho.LR is substantially +1 (under this condition, a sound wave reaches the right and left ears in the same phase). In high frequencies, the phases of the sound wave reaching both ears have no correlation, because a wavelength of the sound wave is shorter than a distance between the ears. Accordingly, the coefficient .rho.LR tends to approach to "0" for high frequencies.
In a narrow space, for example, a space inside a motor vehicle, seat positions are unequally distanced from the right and left speakers. Accordingly, the coefficient .rho.LR at each seat position tends to approach -1, because of the reflection of a sound wave, and because of the asymmetry of a sound source and an acoustic space as perceived from each seat position. The coefficient .rho.LR was measured in the condition that a car driver hears the sounds from only the right and left front door speakers by using a microphone of a dummy head, at a driver's seat in a motor vehicle of the right-hand steering type. The results of the measurement is as shown in FIG. 6. As clearly seen from the graph, the coefficient .rho.LR changes from positive values to negative values in the low and medium frequency regions (the phase of the sound waves at both ears is inverted). This would cause uncomfortable sounds, such as dangling of sound and unclearness of localization.
To correct an acoustic field attended with such an uncomfortable sound, all-pass filters of the second order, by convention, are inserted in the audio signal lines of the right and left channels, respectively. In this case, the all-pass filters are selected so as to have different frequency characteristics as shown in FIG. 7(a). With the different frequency characteristics, a phase difference between the sound waves of the right and left channels is shaped as shown in FIG. 7(b) Accordingly, the coefficient .rho.LR can be improved to be much better than 0 (zero) even in the medium frequency region. A sound is thus dislocated to the front or toward the listener.
The sound field correcting device using the all-pass filters, however, requires complicated filters and hence is expensive to manufacture.